This invention relates to a modulator and demodulator apparatus (modem) which modulates and demodulates a signal in a main channel for main data and in a secondary channel for secondary data obtained by frequency division. More particularly, this invention relates to a modulator and demodulator apparatus of the type mentioned which includes a carrier detector for detecting a carrier from a demodulation signal and executing a data processing sequence for a demodulation signal in accordance with a detection by the carrier detector.
FIG. 12 shows a general construction of an on-line system. Referring to FIG. 12, in the on-line system shown, a plurality of modems 203 are connected to a host computer 201 by way of a communication control apparatus (CCP) 202. Each of the modems 203 is connected by way of an analog circuit or transmission lines 204 to a corresponding one of other modems 203' installed at another location. A terminal 205 is connected to each of the modems 203'.
The on-line system further includes a network supervisory apparatus 206, for which a secondary channel is used.
A state signal of a modem can be transmitted, from each of the host side modems 203 shown in FIG. 12, straight to the network supervisory apparatus 206. However, from each of the terminal side modems 203', a state signal thereof is transmitted to the associated host side modem 203 and then to the network supervisory apparatus 206.
Since a state signal of a modem must necessarily be transmitted without having any influence on main data, each of the modems 203 and 203' divides, for example, a voice band of 0.3 kHz to 3.4 kHz by frequency division. This provides a secondary channel for secondary data and a main channel for main data as seen in FIG. 13.
Phase shift keying (PSK), orthogonal amplitude modulation (QAM) or some other modulation is used for a main signal while frequency shift keying (FSK) is used for a secondary signal.
In each of the modems 203 and 203' described above, main data and secondary data are demodulated separately, and each of demodulation signals is input to a roll-off filter (band separation filter) 207 as shown in FIG. 14. The roll-off filter 207 only passes a signal within a predetermined frequency range of the demodulation signal (a digital output of a demodulator not shown). The roll-off ratio of such roll-off filter 207 is set to one of such values as 15%, 50% and 100%.
The signal having passed the roll-off filter 207 is sent as data to an automatic gain control section (AGC), not shown, or the like. Additionally, it is also input, as shown in FIG. 14, to a carrier detection section (CD section) 208. The carrier detection section 208 detects a carrier from the signal having passed the roll-off filter 207 to detect whether or not data have been received. A result of such detection (CDI-ON/OFF: presence/absence of a carrier) is supplied as trigger information to a sequencer not shown so that a data processing sequence of the demodulation signal is executed by the sequencer. The CDI-ON (carrier presence) signal is a detection signal output from the carrier detection section 208 when a carrier is present whereas the CDI-OFF (carrier absence) signal is a detected signal output from the carrier detection section 208 when no carrier is present.
Where a roll-off filter having a high roll-off ratio such as 15%, 50% or 100% is employed, the convergence of the waveform response is rapid and the time width of the waveform response of the roll-off filter 207 is approximately 10 symbols (one symbol requires one modulation time), as shown in FIG. 15(a). Further, the dispersion time width of the CDI-ON/OFF signal, which is a detection signal from the carrier detection section 208, is approximately 5 symbols, as shown in FIG. 15(b).
In recent years, modems have been required to establish multiple point connection in addition to increasing the communication rate in order to reduce the cost of the circuit. To this end, it is an effective technique to divide a frequency band of a main channel into a plurality of bands to transmit a plurality of data by way of the same circuit. An apparatus which is used for modulation and demodulation of a signal communicated in a main channel having a plurality of frequency bands obtained by frequency division is called a multiple frequency modem.
When a main channel is divided into a plurality of bands in this manner, since adjacent frequencies with reduced band widths must necessarily be distinguished rigidly from each other, the frequency cutoff characteristic of the roll-off filter must necessarily be steep. To this end, the roll-off ratio of the roll-off filter of the multiple frequency modem is set very low (for example, 3 to 5%). When, for example, a roll-off filter, having a roll-off ratio of 4.5%, is used as the roll-off filter 207, the time width of the filter waveform response is approximately 60 symbols as shown in FIG. 16(a). Consequently, the dispersion time width of the CDI-ON/OFF signal, which is a detection signal from the carrier detection section 208, is approximately 30 symbols, as shown in FIG. 16(b).
Meanwhile, a modem is constructed such that, upon transmission of data, training data of a particular pattern are modulated and transmitted prior to transmission of the data. Such training data are demodulated by demodulation means. Further, initialization processing (training) of a reception section of the modulator and demodulator apparatus is performed using the demodulation training data.
Since the time required for such training (such time is called "RS (request-to-send)--CS (clear-to-send) time) is approximately 18 symbols, the dispersion time width of the CDI-ON output of the carrier detection section 208 is important. In some cases, at a point in time when a carrier is detected by the carrier detection section 208, training data may not be received and a data reception condition may be established although initialization processing has not been performed as yet.
Further, when polling is performed, if the CDI-OFF output of the carrier detection section 208 disperses significantly so that the output is delayed, then a next carrier may not possibly be received normally.